Many MEMS devices require that their mechanical resonant frequencies are matched to system requirements or to other on-chip frequencies. Examples of these requirements are UHF resonators for RF signal processing, channel selection, and “on-resonant” operation of arrays of resonators for chemical or biological sensors. Each resonator should be tuned to the pre-selected frequency used in order to properly receive the signal. In the case of resonators to be used with chemical or biological sensors, an array of resonators with frequencies in the 100 MHz to 1 GHz range should ideally have nearly matched resonant frequencies since differential detection of small changes in the frequencies due to molecular attachment are more easily measured without large offsets in frequency. In some digital radio applications, thousands of individual channels in the 100 MHz range should be matched to the broadcast frequencies with an accuracy of several kilohertz. Since fabrication tolerances are typically about 1% for most IC fabrication techniques, processing errors from etching and lithography techniques can far exceed the resonant linewidths or channel accuracy required for high Q oscillators. Therefore, etching and lithography do not provide the accuracy necessary for high frequency MEMS resonators.
Previously, most MEMS resonators have been tuned using electrostatic biases applied to the resonators, thus lowering the resonant frequency through a negative spring effect. This technique has been used for a nickel (Ni) ring gyro reported at the “Solid-State Sensor and Actuator Workshop,” Hilton Head, S.C., Jun. 13-16, 1994, pp. 213-220.
However, the electrostatic bias technique has a limited tuning range since the bias voltages applied to the resonators deflect the structure of the resonators and cannot be raised to high levels due to limitations on the electronics. Additionally, this biasing technique can lead to drift over time and temperature, if the biasing source changes with time or temperature. Furthermore, the electrostatic bias technique can lead to additional energy losses to the support circuitry and therefore a low overall Q of the system. Moreover, electrostatic tuning cannot mass balance a system to minimize momentum losses to the support structure. Hence, a method to mechanically tune a MEMS resonator both for frequency adjustment and for Q optimization is desired.
In order to solve the problems associated with tuning MEMS devices using electrostatic biases, laser assisted etching techniques have been utilized to mechanically tune MEMS gyros. The Draper double tuning fork gyro is an example of a device tuned with this technique. However, laser assisted etching has less spatial resolution for nanometer structures and cannot control the removal of material on a sub-monolayer scale. In addition, laser systems can cause damage or drift of the structure due to heating effects, and require additional equipment for accurately removing material on the micron scale. This is discussed in M. Weinberg, J. Connely, A. Kourepenis, D. Sargent, “Microelectromechanical Instrument and Systems Development at the Charles Stark Draper Laboratory, Inc.” Also, laser assisted etching and chemically assisted etching systems can produce debris which may prevent the MEMS structure from moving or cause reactions with metals or other device features of the MEMS structure. This is discussed in Amy Duwell, Marcie Weinstein, John Gorman, Jeff Borenstein, Paul Ward, “Quality Factors of MEMS Gyros And The Role Of Thermoelastic Damping.”
In general, for commercial high volume applications, the tuning and adjustment time must be on the order of 1 second for high throughput. If the adjustment time is significantly larger than 1 second per device, the cost of manufacturing will be increased. Since air damping can affect the parameters of MEMS resonators, the tuning technique must be performed in a reasonably good vacuum (10−4 to 10−6 Torr) for most MEMS devices. Therefore, there is a need for a method which can reliably tune many MEMS devices in a short period of time in a vacuum. This can be best accomplished by tuning a small or “nano” MEMS device on a wafer before packaging, using a sub-micrometer vector scan beam system emitting a focused ion beam.
Therefore, a need arises to provide a method which can reliably and cleanly tune a MEMS device in real time with sub-micron accuracy. Furthermore, there is a need for a method which can reliably tune many MEMS devices in a short period of time in a vacuum.